Construction machines, such as hydraulic excavators and hydraulic cranes, perform various work by means of a hydraulic drive system. For example, Patent Literature 1 discloses a hydraulic drive system of a hydraulic excavator, which is configured such that hydraulic oil is supplied from a first pump and a second pump to a plurality of actuators via a plurality of control valves.
Specifically, in the hydraulic drive system disclosed in Patent Literature 1, a plurality of control valves including a boom control valve are disposed on a first circulation line extending from a first pump to a tank, and a plurality of control valves including a turning control valve are disposed on a second circulation line extending from a second pump to the tank. The first pump and the second pump are variable displacement pumps. The tilting angle of the first pump is changed by a first regulator, and the tilting angle of the second pump is changed by a second regulator.
Each of the first regulator and the second regulator includes a first servo valve for use in positive tilting control and a second servo valve for use in total horse power control. The first servo valve moves in accordance with a secondary pressure outputted from a first solenoid proportional valve, and the second servo valve moves in accordance with the discharge pressure of the first pump, the discharge pressure of the second pump, and a secondary pressure outputted from a second solenoid proportional valve.
Patent Literature 2 discloses a hydraulic drive system of a construction machine, which is configured to reduce a relief amount at the start of turning. Specifically, in the hydraulic drive system, a running control valve, a turning control valve, an arm control valve, a boom control valve, and a bucket control valve are disposed on a circulation line extending from a single variable displacement pump to a tank. The circulation line is provided with a pressure meter that measures the discharge pressure of the pump. The tilting angle of the pump is changed by a regulator, and a high pressure selective valve is connected to the regulator. The high pressure selective valve leads a higher one of the following pressures to the regulator: a negative control pressure, which is the pressure at the upstream side of a throttle provided on the circulation line; and a secondary pressure from a solenoid proportional valve. The solenoid proportional valve is controlled by a controller. The controller feeds a command current to the solenoid proportional valve for a predetermined time when a turning operation is performed and the amount of change in the discharge pressure of the pump has increased rapidly. As a result, a high secondary pressure is outputted from the solenoid proportional valve, and the discharge flow rate of the pump is suppressed temporarily. Consequently, the relief amount at the time of starting a turning motor is reduced.